Kenneth



ELECTRICAL CAPACITORS Filed Aug. 6, 1956 AL\\ \\\\\\\\\Yq INVENTOR KENNETH G1). WILLIAMS lBy owm 6,4,, M war- 4m ATTORNEYS United States Patent ELECTRICAL CAPACITORS Kenneth George Daniel Williams, London, England, as-

signor to A. H. Hunt (Capacitors) Limited, London, England, a British company 7 Application August 6, 1956, Serial No. 602,211

Claims priority, application Great Britain August 8, 1955 9 Claims. (Cl. 2925.42)

This invention relates to electrical capacitors, and more particularly to those in which the dielectric material is polyethylene terephthalate. Polyethylene terephthalate is a synthetic plastic polymer possessing electrical characteristics which make it suitable for use as a dielectric, and it is known that the electrical characteristics can be improved by subjecting it to a relaxing heat treatment.

According to the present invention, in the manufacture of an electrical capacitor with polyethylene terephthalate dielectric, after assembly of the dielectric and electrodes the unit is subjected to drying below the relaxation temperature of the dielectric followed by a heat treatment above said temperature. The drying should be carried out below 150 C., and preferably the unit is dried under vacuum at about 50 C. The heat treatment is preferably carried out at 200 C., and may be of 1 /2 hours duration. 7

I have found that the heat treatment can give approximately a ten-fold improvement in the electrical resistance of the dielectric, but this improvement is only obtained if the drying is carried out first.

According to a further aspect of the invention, the drying and heat treatment steps may be combined-into a single operation in that the assembled capacitor unit with polyethylene terephthalate dielectric may be subjected to treatment in which the unit is placed under vacuum or in a non-oxidising atmosphere and taken slowly up to a temperature of 180 C. to 200 C., and afterwards allowed to cool still under vacuum or in the non-oxidising atmosphere. Preferably the duration of the heating treatment is about two hours.

Various methods of carrying the invention into effect will now be given by way of example, reference being bad to the accompanying drawings, Figures 1 and 2 of which both illustrate the first method to be described.

In the first method a roll capacitor unit is wound in known manner from thin flexible dielectric strip material bearing metallisation to constitute the electrodes, for example in the manner described in British patent specification No. 563,080. The dielectric is however polyethylene terephthalate film of the kind marketed under the registered trademark Melinex. Both end faces of the roll are then repeatedly slashed in criss-cross fashion with a fine cutter blade, the depth of the cuts being only a few thousandths of an inch; this is to assist the penetration of the copper spray, to be later employed for making the terminal connections, as described more fully in British patent application No. 22,761/55.

The unit is next vacuum dried at 50 C. for fifteen hours followed by heat treatment at 200 C. for 1 /2 hours. The duration of the heat treatment is quite critical. Afterwards the ends of the roll are sprayed with copper and the terminal connections made.

It is found that during the heat treatment the edges of the dielectric at the ends of the roll tend to curl to some extent, first inwardly toward the centre of the roll and then out. Figure 1 shows in cross section two contiguous dielectric strips 11, 12 in a capacitor roll, which bear metallisation 13 and 14, these two strips having demetallised margins 15 on opposite sides (only the demetallised margin of the strip 11 being shown) and being assembled in the manner described in British patent specification No. 563,080. The strips 11, 12 are in staggered arrangement, with the metallised edge 16 of one projecting beyond the unmetallised margin 15 of the other on each side. In Figure 1, the strips are illus trated in their fiat condition before the heat treatment and Figure 2 shows the same two strips after heat treatment where it will be seen how the projecting metallised edge 16 on the dielectric strip 12 has first curled in at 17 toward the centre of the roll and then out again at 18.

Consequently, when the electrode metallisation to which a terminal connection has to be made is on the inside face of the dielectric strip as shown, the metallisation on the edge 16 is turned out and presented to the metal spray by this curling. The slashing of the ends of the roll and the edge curling brought about by the heat treatment therefore combine to ensure that mechanically secure terminal connections are obtained, and this is one reason why the copper spraying is not carried out until after'the heat treatment.

Another reason why the copper spraying is delayed until after the heat treatment is the fact that the copper would otherwise tend to oxidise during said treatment. However, if it is particularly desired to spray before heat treatment, this latter difiiculty may be overcome, for example, by tinning the copper, or by carrying out the heat treatment in a non-oxidising atmosphere as will hereinafter be described.

Although the figure of 200 C. is mentioned herein for the heat treatment, it is believed that a satisfactory improvement in the properties of'the dielectric can be obtained at temperatures below this, down to say 180 C., although somewhat longer periods of treatment would no doubt be required. The removal of water from the plastic dielectric before heat treatment, by the drying step below the relaxation temperature, is essential, as otherwise the plastic becomes brittle during the heat treatment and breaks up.

In another method of carrying the invention into effect, a roll capacitor unit is wound from metallised dielectric of polyethylene terephthalate, as before. The opposite ends of the roll so formed are then sprayed with copper to afiord the terminal connections, that is to say in this case the copper spraying is carried out before any heat treatment.

After spraying the unit is placed under vacuum, and is heated gradually up to a temperature of 180 C., the time taken to reach this temperature being 2. hours. At the end of the 2 hour period the unit is allowed to cool off while still being held under vacuum. The unit may then be finished off in the usual Way by soldering terminal wires to the sprayed copper at each end, testing and so forth.

In this method, since the heat treatment iscarried out under vacuum, there is no tendency for the copper already sprayed on to the ends of the roll to oxidise. It

will be appreciated that, in this instance, it is not pos-' after heat treatment. This, followed by shrinking of the roll during the subsequent heat treatment, ensures a mechanically secure terminal fixing without any special steps having to be taken.

During the heat treatment by either method the dielectric is subject to an appreciable degree of shrinkage and the unit as a whole contracts. This has the effect that if the unit is wound on a small diameter winding spindle the small central hole left through the unit by the Winding spindle becomes very minute after the heat treatment and may disappear entirely. One result or this is that if the hole disappears it does not need to be plugged or masked during metal-spraying of the roll ends, as would otherwise be necessary to stop the sprayed metal entering the hole.

A further result is that if the terminal wires for the capacitor are pushed into opposite ends of the central hole before the heat treatment, the contraction of the hole during heat treatment, causes the wires to be gripped, and they are thereby held ready for soldering to the roll ends after these have been sprayed. This ensures that the terminal wires are attached centrally to the unit, which is a particular advantage when these wires are to be used for locating the capacitor unit in a mould ready for a resin casing to be moulded round it, as described in British eaten; application No. 10,875/54 and Cognate No. 17,276/54.

I claim:

1. A method of manufacturing an electrical capacitor with polyethylene terephthalate dielectric, wherein after assembly of the dielectric and electrodes, the unit is subjected to drying below the relaxation temperature of the dielectric followed by a heat treatment above said temperature both of which steps are carried out under non-oxidizing conditions.

2. A method as claimed in claim 1 wherein the drying is carried out under vacuum at about 50 C, and the heat treatment is carried out at 200 C. or thereabouts.

3. A method as claimed in claim 2 wherein the duration of the drying is about 15 hours, and that of the heat treatment about 1 /2 hours.

4. A method as claimed in claim 1 wherein the dry- 4 ing and heat treatment steps are combined into a single operation in that the capacitor unit is taken slowly up to a temperature of about 180 C., to 200 C. under vacuum over a period of several hours.

5. A method as claimed in claim 4 wherein the period for raising the temperature of the unit to 180 C. to 200 C. is 2 hours or thereabouts, and afterwards the unit is allowed to cool still under vacuum.

6. A method as claimed in claim 4, wherein the capacitor unit is of the roll type and the roll ends are aycd with metal, such as copper, to secure terminals, before the unit is subjected to the heat treatment.

7. A method as claimed in claim 1 wherein the drying and heat treatment steps are combined into a single operation by slowly heating the capacitor to a temperature the range of 180 C. to 200 C. in a non-oxidisatrnosphere for a period of several hours.

8. A method as claimed in claim 7 wherein the period for heating said capacitor is of the order of two hours, further including the step of cooling the capacitor in a non-oxidising atmosphere.

9. A method of manufacturing an electrical capacitor with polyethylene terephthalate dielectric comprising the steps of winding electrodes and dielectric material on a small diameter spindle, removing said spindle, inserting terminal wires in opposite ends of the capacitor in holes left by said spindle, drying said capacitor at a temperature below the relaxation temperature of said dielectric material and heat treating said capacitor at a temperature above said relaxation temperature, said wires being maintained in position during the heat treatment, both the drying and heat treating being carried out under nonoxidizing conditions whereby the dielectric material shrinks under the treatment and grips said wires.

References Cited in the file of this patent UNlTED STATES "PATENTS 2,676,124 Foster Apr. 20, 1954 2,797,373 Peck June 25, 1957 FOREIGN PATENTS 893,031 France May 26, 1944 584,549 Great Britain Jan. 17, 1947 

1. A METHOD OF MANUFACTURING AN ELECTRICAL CAPACITOR WITH POLYETHYLENE TEREPHTHALATE DIELECTRIC, WHEREIN AFTER ASSEMBLY OF THE DIELECTRIC AND ELECTRODES, THE UNIT IS SUBJECTED TO DRYING BELOW THE RELAXATION TEMPERATURE OF THE DIELECTRIC FOLLOWED BY A HEAT TREATMENT ABOVE SAID TEMPERATURE BOTH OF WHICH STEPS ARE CARRIED OUT UNDER NON-OXIDIZING CONDITIONS. 